Photosynthesis: Sites, Stages & Light Reaction

Nutrition Modes in Organisms

  • Organisms use different energy sources for nutrition, such as light, organic molecules, or inorganic molecules.
  • Based on energy and carbon sources, organisms are classified as autotrophs or heterotrophs.
  • Autotrophs are self-feeders (producers) like plants and algae, occupying the first trophic level.
  • Heterotrophs depend on other organisms for food.

Types of Autotrophs

  • Photoautotrophs: Use light energy (e.g., plants, algae, cyanobacteria) to synthesize organic molecules from carbon dioxide via photosynthesis.
  • Chemoautotrophs: Use chemical energy (e.g., hydrogen sulfide, ammonia) to synthesize food; unique to certain prokaryotes. Examples: sulfur bacteria.

Photosynthesis

  • Net equation: CO2 + H2O + Light Energy \rightarrow C6H{12}O6 + O2
  • Carbon dioxide is reduced to glucose, and water is oxidized to oxygen.
  • Photosynthesis is the reverse of cellular respiration.

Chemosynthesis

  • Organisms (bacteria) use chemical energy to produce carbohydrates.
  • Utilize inorganic compounds (e.g., hydrogen sulfide, sulfur, ammonium) as reducing agents to synthesize organic compounds from CO_2.
  • Found in hostile environments like deep sea vents.

Photosynthesis in Plants

  • Photosynthetic eukaryotes with chloroplasts follow the same process.
  • Leaves contain mesophyll cells with chloroplasts, the organelles containing green pigment.
  • Stomata are conduits for gas exchange, allowing CO2 entry and O2 release.

Chloroplast Structure

  • Double membrane structure: outer membrane, intermembrane space, inner membrane.
  • Thylakoids: Disc-like structures stacked into grana; contain chlorophyll.
  • Stroma: The matrix surrounding the thylakoids.

Photosynthetic Pigments

  • Compounds that absorb sunlight (photons).
  • Chlorophyll a: Main pigment, absorbs violet, blue, and red light, reflects green light.
  • Chlorophyll b: Accessory pigment, transfers absorbed light energy to chlorophyll a, extending the range of wavelengths absorbed.
  • Carotenoids and Phycobilins: Other pigments present.

Chlorophyll Types

  • Chlorophyll a: Essential for oxygen-generating photosynthesis in all photosynthetic organisms and cyanobacteria.
  • Chlorophyll b: Present in plants and green algae; transfers light to chlorophyll a.
  • Chlorophyll c: Present in brown algae and diatoms instead of chlorophyll b.
  • Photosynthetic bacteria contain bacteriochlorophyll or chlorobium chlorophyll; do not produce oxygen.